The desirability of enhancing the growth of crop plants and, thereby, increasing the efficiency of food and fiber production is well established. The search for methods to enhance plant growth goes back nearly to the beginning of civilization and continues to be an important endeavor. The use of chemicals other than plant nutrients to enhance plant growth is more recent, but well established in principle. Chemicals having the property of enhancing the growth of crop plants and, especially, of increasing the yield of crop plants are, therefore, valuable, and the discovery of new classes of chemicals having this property is highly desirable.